1. Field of the Invention
The present invention relates to a filter device, and particularly to a filter device suitable for application to transmission power control or the like used in wireless communications. The present invention also relates to a transmission power control apparatus, and particularly to a transmission power control apparatus suitable for application to a transmission power control station or the like for controlling transmission power of a device to be controlled, based on control information about power.
2. Description of the Related Art
A wireless communication system performs communications in the best communication state between a communication terminal equipment, i.e., a mobile station that performs communications while being moved, and a base station to thereby provide communication services. The base station and the mobile station monitor received power or the quality thereof each other so as to perform suitable transmission according to the distance therebetween. The wireless communication system operates so as to notify both stations of the resultant results of monitoring each other to thereby form a feedback loop related to transmitting power control. In this case, the wireless communication system is capable of suppressing power consumption as compared with the case in which communications are made at a constant power, and makes it possible to lengthen the time required to use a carrying mobile station.
In such a wireless communication system, there may be cases in which a transmission error occurs in communications between the base station and the mobile station so that transmission power cannot be always most suitably supplied. Therefore, the quality of communications cannot be maintained. Thus, a measure against such a case has been proposed in each of a transmission power control method, a base station apparatus and a communication terminal equipment described in Japanese Unexamined Patent Publication No. Hei 11(1999)-196042. This measure resides in that when a transmission power value is controlled in the direction in which the indicated contents of a received control signal exceed a power control range, in a state in which the transmission power value falls within a limit value of the power control range, the number of times that the control signal is received is counted, and when the power value is controlled in the direction not to cause the subsequent indicated contents to exceed the range, the count value of the number of times that the control signal is received, is reduced, and only when the count value has reached a predetermined value, the power is controlled in the direction not to cause the contents to exceed the power control range. Owing to this measure, the base station and the mobile station in the wireless communication system do not perform power control immediately even where they receive control of power in a wrong direction due to an error or the like, and control transmission power only when a normal control signal is received a predetermined number of times, thereby always maintaining the optimum transmission power.
The control of the transmission power by the wireless communication system will be explained in brief. Control information about transmission power communicated between the base station and the mobile station is determined based on a power control renewal cycle in which the control information is transmitted according to specs inherent in the system, and the amount of gain control by one control information, i.e., the amount of transmitting power control. As a specific example thereof, may be mentioned specs based on W-CDMA (Wide band-Code Division Multiple Access). According to the W-CDMA specs, it is provided that the power control renewal cycle is a 2560 chip, and the amount of transmitting power control takes or adopts either one of 0.5,1,1.5 and 2 dB. The present provision is provided to suppress a vibrational phenomenon of the transmission power due to the transmitting power control.
Meanwhile, in a system wherein more precise transmission power control is performed, a power control renewal cycle is shortened on the side of a transmission power control station where transmission power is generated so that a feedback response becomes quick. However, the shorter the renewal cycle with it, the more a vibrational phenomenon of a transmit signal tends to occur noticeably. Although the tendency to allow easy occurrence of oscillations differs according to a control information extracting method in the transmission power control station, a system using the extracting method having such a strong tendency might cause vibrations in transmit signal.
As a countermeasure against it, there is known a method of changing respective set values used in the transmission power control station. However, values, which are provided in advance by the specs inherent in the system and are not able to change, also exist in the set values. As examples of the unchangeable values, may be mentioned, the amount of transmitting power control by power control information, and a power control information renewal cycle. As information signals like these, only certain specific values are restrictively set according to interface specs between transmission and reception.
In addition to it, a method of reducing loop gain in a feedback loop is known as a method of suppressing vibrations in transmit signal by power control. In the case of the transmission power control provided by the specs, however, the amount of transmitting power control and the power control information renewal cycle correspond to loop gain. There may be cases where the amount of transmitting power control and the power control information renewal cycle are set as the unalterable values as mentioned above. Accordingly, a loop gain-based adjustment might not be applied as the method of suppressing the vibrations in transmit signal.
As apparent from these reasons, the optimum control in a wireless communication system of W-CDMA or the like involves a problem that the fact that it cannot be done in accordance with communication circumstances of the system under the present situation, could happen.